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The principle of most modern day clean rooms is based
upon laminar flow, or to be more precise unidirectional airflow. Parallel
streams of air, moving at a uniform velocity of 0.3 to 0.45 metres per second
are blown across the room with as little turbulence as possible. The principle
is based on expelling any dust in the airflow by the shortest route. In general,
the greater the frequency with which the air is renewed, the lower the
contamination build up in the room and the greater the rate at which the
impurities are diluted
The most common types of clean room found today are
either vertical down flow or horizontal cross flow. In a vertical down flow
clean room, air enters the room via filters in the roof and exits through vents
in the floor. Although this design has proven to be technically efficient and
can attain the highest of specifications, it is extremely costly in both capital
and maintenance.
In a horizontal cross flow clean room, filtered air
enters the room from one side wall and is exhausted above the floor
in the sidewall opposite and/or re-circulated via a bank of filters. The air
velocity has to be established at a sufficiently high level to ensure that dust
particles do not thermally migrate from the laminar flow.
The design of a clean room has to accommodate many
factors including:
Process - what is the client intending to use the room
for?
Longevity - how long is the room expected to last?
Traffic - how many times will personnel enter /
leave the room per hour/ day?
Heat loading - what heat is equipment / personnel
outputing to the room?
Air extraction - how much air is being extracted
for processes?
Air conditioning - is comfort cooling / heating
required or something more stringent? What ambient max and min temperature
parameters are to be specified?
Humidity - does this
need to be controlled and if so within what parameters?
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